A modular multi-stage converter, also called a Modular Multilevel Converter or MMC or M2C for short, converts a first voltage into another voltage. A modular multi-stage converter can convert a DC voltage into an AC voltage for example, i.e. it is used as an inverter. A modular multi-stage converter can moreover convert an AC voltage into a DC voltage for example, i.e. it is used as a rectifier. Furthermore, a modular multi-stage converter can convert a first AC voltage into a second AC voltage with a different frequency and amplitude, without first of all creating a DC voltage, i.e. it is used as a direct converter.
Modular multi-stage converters are primarily used in high-power applications. An inverter may be used in a photovoltaic power plant, in which the DC voltage generated by the power plant must be converted into an AC voltage, in order to be able to feed said AC voltage into an alternating current network. A modular multi-stage converter is needed as a rectifier for example in high-voltage direct current (HVDC) transmission. In such systems a DC voltage, which is needed for low-loss transport over long transmission routes, is created from an AC voltage. After its transmission the DC voltage is again converted into an AC voltage with a modular multi-stage converter as an inverter, in order to feed said voltage into an alternating current network.
In its structure a modular multi-stage converter typically comprises at least one series circuit with a number of modules connected in series. EP 2677653 A1 discloses a modular multi-stage converter which comprises three series circuits with 2N modules. Each of the series circuits is divided by an AC voltage line into two arms. Each arm in its turn is connected via a choke element to the AC voltage line. Each module also has an energy store and a switching device. The respective energy store is configured to store electrical energy. In each module the energy store can be charged at least partly with electrical energy. With the aid of the switching device it can be controlled whether the energy store of the respective module is to be charged or discharged or whether the energy store is to be bypassed.
The switching devices of the modules are activated by the control device for the multi-stage converter. For this and for feeding back measured values for charging of the respective energy store, bidirectional communication connections between the modules and the control device are necessary. These are typically realized in high-power applications as point-to-point optical fiber connections. These are expensive, however, because of the quantity of connections between the control device and each of the modules. As an alternative, electrical bus connections may be used. With these a suitable electrical insulation between the bus and the individual modules has to be insured, which is likewise complicated and expensive.